Maraging steel

ABSTRACT

Provided is maraging steel with which maraging steel that has been subjected to nitriding treatment has excellent fatigue strength and stable high strength. Maraging steel that has been subjected to nitriding treatment to a thickness of 0.5 mm or less and comprises, by mass %, 0.008% or less of C, 0.5% or less of Si, 0.5% or less of Mn, 0.5 to 1.5% of Cr, 16.0 to 22.0% of Ni, 3.0 to 7.0% of Co, and 3.0 to 7.0% of Mo is maraging steel further comprising, by mass %, 0.6 to 1.3% of Al and Fe and impurities as the balance, wherein the impurities are maintained at 0.01% or less of Ti, 0.01% or less of Nb, 0.01% or less of Ta, and 0.01% or less of W, the ratio of Al and Ti is Al/Ti&gt;250, and by Vickers hardness, the surface hardness of the maraging steel is 800 to 1,050 HV and internal hardness of the maraging steel is 570 HV or less.

TECHNICAL FIELD

The present invention relates to maraging steel subjected to nitridingtreatment that is preferable to a metal endless belt for a CVT(Continuously Variable Transmission).

BACKGROUND ART

The maraging steel has a very high tensile strength of about 2000 MPaand thus is used for a metal endless belt for a CVT or the like, whichrequires a high strength, by being machined to a steel strip having athickness of 0.5 mm or less. The maraging steel typically has a chemicalcomposition of 18% Ni, 8% Co, 5% Mo, 0.45% Ti, 0.1% Al, and the balanceof Fe, by mass %.

However, the above-described maraging steel has not always a highfatigue strength, while it exhibits a very high tensile strength. Thebiggest factor of degrading the fatigue strength of the maraging steelis TiN inclusion. Since the TiN inclusion tends to be large in size andhas a cubic shape, fatigue fracture originating from the inclusion tendsto occur.

For this reason, there is proposed maraging steel to which Ti is notadded. In the maraging steel subjected to the nitriding treatment forthe metal endless belt, Ti is an important element for not onlyincreasing the strength of a base (matrix) of alloy, but also increasingthe strength of a nitrided layer. If Ti is not added, the strength ofboth base and nitrided layer would be reduced. Therefore, in themaraging steel to which Ti is not added, it is required to reinforce thebase and the nitrided layer with an element other than Ti.

As a first method for reinforcing the base and the nitrided layer, thereis a method of using Cr to reinforce the nitrided layer, whileincreasing the contents of Co and Mo which contribute to reinforce thebase. The first method is disclosed in JP-A-2009-013464 (PatentLiterature 1), JP-A-2008-088540 (Patent Literature 2), JP-A-2007-186780(Patent Literature 3) and WO2009/008071 (Patent Literature 4) by thesame applicant as the present invention, for example.

As a second method, there is a method of reducing Co, as well as Ti, andpositively adding any one or more of Al, Si and Mn. This second methodis disclosed in JP-A-2001-240943 (Patent Literature 5) andJP-A-2001-240944 (Patent Literature 6) by the same applicant as thepresent invention, for example. The above-described patent literaturespropose maraging steel having a high fatigue strength, containing, bymass %, 0.008% or less of C, 2.0% or less of Si (including 0), 3.0% orless of Mn (including 0), 0.010% or less of P, 0.005% or less of S, 12to 22% of Ni, 3.0 to 7.0% of Mo, less than 7.0% of Co, 0.1% or less ofTi, 2.0% or less of Al, 8.0 to 13.0% of 3Si+1.8Mn+Co/3+Mo+2.6Ti+4Al,less than 0.005% of N, 0.003% or less of O, and the balance beingsubstantially Fe, and further containing, as optional elements, 4.0% orless of Cr, 0.01% or less of B, 1.0% or less of Nb, 2.0% or less of Taand 2.0% or less of W.

CITATION LIST Patent Literature Patent Literature 1: JP-A-2009-013464Patent Literature 2: JP-A-2008-088540 Patent Literature 3:JP-A-2007-186780 Patent Literature 4: WO2009/008071 Patent Literature 5:JP-A-2001-240943 Patent Literature 6: JP-A-2001-240944 SUMMARY OFINVENTION Technical Problem

The maraging steel according to the above-described Patent Literatures 1to 4 contain expensive Co in the range of more than 7.0%. Co is a raremetal, and it is feared that the price of Co raw material, is raised inthe future.

Meanwhile, in the maraging steel described in the Patent Literatures 5and 6, the contents of both Ti and N are kept as low as possible for thepurpose of reducing TiN inclusions that are adverse to improvement infatigue strength. Further, in order to make the maraging steel lessexpensive, the content of Co is set to less than 7.0% and the additionalelements are added by appropriate amounts. As a result, both highstrength and high fatigue strength can be achieved at low price.

However, in applications such as a metal endless belt for a CVT and thelike, it is required to stably achieve an excellent strength, inparticular, by maximizing the effect of precipitation enhancement bynitride formation after a nitriding process. The strength of theconventional maraging steel according to the above-described secondmethod does not necessarily satisfy such a high demand, and therefore,it is required to achieve more excellent fatigue strength and stablyhigh strength.

An object of the present invention is to provide maraging steelsubjected to nitriding treatment, having an excellent fatigue strengthand stably providing a high strength.

Solution To Problem

The present inventors have studied an optimal composition whichmaximizes precipitation enhancement by nitride formation after thenitriding treatment and stably provides an excellent strength, based onthe maraging steel described in the Patent Literatures 5 and 6. As aresult, it has been found that Si and Mn, which have been conventionallyadded for the purpose of improvement in strength, should beadvantageously reduced in order to ensure good toughness and ductility.

Moreover, the present inventors have found, from their renewedinspection of the effect of Al which contributes to improvement instrength, that high strength and fatigue strength characteristics can beobtained by regulating the Al content to be in a specific range andaccordingly optimizing the composition of the steel and regulating thehardness. Eventually, the inventors have reached to the presentinvention.

That is, the present invention provides maraging steel subjected tonitriding treatment, containing 0.008% or less of C, 0.5% or less of Si,0.5% or less of Mn, 0.5 to 1.5% of Cr, 16.0 to 22.0% of Ni, 3.0 to 7.0%of Co, and 3.0 to 7.0% of Mo by mass % and has a thickness of 0.5 mm orless, wherein the maraging steel further contains 0.6 to 1.3% of Al bymass %, with the balance being Fe and impurities, the impurities beingrestricted to 0.01% or less of Ti, 0.01% or less of Nb, 0.01% or less ofTa, and 0.01% or less of W, and a ratio of Al to Ti satisfies Al/Ti≧250,and a surface hardness of the maraging steel is 800 to 1,050 HV and aninternal hardness of the maraging steel is 570 HV or less by Vickershardness.

The above-described Al content is preferably 0.7 to 1.2% by mass %.

Further, the present invention may further contain, in addition to theabove-described compositions, either 0.0001 to 0.0030% of Mg or 0.0001to 0.0030% of Ca, or both by mass %. Further, the present invention isthe maraging steel which may further contain, in addition to theabove-described compositions, 0.0001 to 0.0030% of B by mass %.

Advantage Effects of Invention

The maraging steel according to the present invention achieves asuitable sensitivity to a surface defect of the maraging steel afternitriding treatment and provides high strength and fatigue strengthcharacteristics by optimizing Al content and accordingly optimizing thecomposition of the maraging steel and regulating the hardness.

Therefore, the maraging steel according to the present invention hasvery excellent mechanical properties, such as long fatigue life or thelike when it is used for a member which requires a high fatigue strengthsuch as a metal endless belt for a CVT.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a view showing the relationship between Al content and tensilestrength.

DESCRIPTION OF EMBODIMENTS

As described above, the characteristics of the present invention aremade in view of the above-described novel finding. The action of Al,which is the most important element in the present invention, will nowbe described. Note that the content of elements is indicated by mass %.

Al: 0.6% to 1.3%

Al is the most important element in the present invention and it hasbeen found from a variety of experiments that it is required to limitthe Al content within a suitable range in order to obtain high strengthand high fatigue strength. In the maraging steel containing reduced Tior the like according to the present invention, Al combines with Ni toform NiAl and Ni₃Al, which are intermetallic compounds necessary toobtain sufficient internal hardness. Therefore, Al has an effect ofincreasing the strength of a base. Al is also advantageous forimprovement in fatigue strength, because AlN precipitated in a meltingand solidification process is finer than TiN, when compared to commonmaraging steel containing Ti. Further, Al is an important elementcapable of forming fine AlN in order to obtain the hardness of thenitrided layer in the nitriding treatment.

However, if the Al content is high, the amount of the intermetalliccompounds becomes excessively high to cause increase in internalhardness, while the amount of the fine AlN becomes also excessively highafter the nitriding treatment to raise the surface hardness too high. Ithas been found that this fact results in higher sensitivity to aninternal defect or a surface defect in the steel, which causes decreasein strength or fatigue strength and leads to variations in strength.

In the present invention, the upper limit of the Al content is thuslimited to 1.3% or less. If the Al content is 1.3% or less, it ispossible to obtain high strength and high fatigue strength because theamount of the intermetallic compounds and the nitride remain within asuitable range. It is more preferable that the upper limit of the Alcontent is set to 1.25% in order to more reliably reduce variations instrength. The more preferable upper limit is 1.2% and the further morepreferable upper limit is 1.15%.

Meanwhile, if the Al content is low, the amounts of the intermetalliccompounds and the nitride as described above are too low to obtainsufficient internal strength and surface hardness after the nitridingtreatment. For this reason, the lower limit of the Al content is set to0.6% or more. The more preferable lower limit of the Al content is 0.7%and the further more preferable lower limit of the Al content is 0.8%.

Furthermore, in the present invention, it has been found that, besidesTi as described hereinafter, Ta, W and Nb disclosed in the maragingsteel described in the Patent Literatures 5 and 6 are also elementswhose level should be limited to be equal to or less than that ofimpurity elements, in order to maximize the above-described effect ofAl. These elements easily combine with C, N, Ni and the like and form acompound to cause decrease in toughness or variations in strength. Thereason will be described hereinafter.

The ratio of Al to Ti is Al/Ti≧250

Next, the ratio of Al to Ti for maximizing the above-described effect ofAl will be described.

As described hereinafter, Ti is an impurity element which is to berestricted in the present invention. Ti is a very active element andtherefore easily combines with N in a melting production to form TiNwhich is a non-metallic inclusion, so that the fatigue strength in thehigh cycle range is decreased. Further, in the maraging steel containing0.6% to 1.3% of Al according to the present invention, if impurity Tiremains excessively, Ni₃Ti which is an intermetallic compound containingTi is formed in heat treatment, so that formation of Ni₃Al which is theintermetallic compound containing Al and is to be otherwise formed isinhibited. Further, since Ti nitrides are preferentially formed in thenitrided layer in the nitriding treatment, fine AlN, which is to beotherwise formed, is not sufficiently formed in the nitrided layer. As aresult, high strength and high fatigue strength can not be obtained.Thus, in the present invention, the ratio of Al to Ti (Al/Ti) ismaintained to 250 or more by mass %. If the ratio of Al to Ti (Al/Ti) isless than 250, the above-described effect of the impurity Ti reaches thenitrided layer to inhibit formation of AlN in the nitrided layer so thatthe effect of Al can not be maximized.

A more preferable range of the ratio of Al to Ti is 300 or more, by mass%. The ratio is more preferably 350 or more, and further more preferably400 or more.

Ti: 0.01% or less

Next, Ti will be described. Ti is originally an important element forreinforcement of the maraging steel, but forms TiN in theabove-described nitrided layer, as well as TiN or Ti(C, N) asinclusions. Therefore, Ti is a detrimental element which decreasesfatigue strength, in particular, in a very high cycle range. For thisreason, in the present invention, it is required to keep the Ti contentas low as possible as an impurity. Further, Ti tends to form a thin andstable oxide film on the surface. When formed, this oxide film inhibitsthe nitriding reaction and therefore it is difficult to obtainsufficient compressive residual stress in the nitrided surface. In orderto facilitate the nitriding and increase the compressive residual stressof the surface after the nitriding, it is required to keep the Ticontent as low as possible. If the Ti content is 0.01% or more,sufficient effect can not be realized for decreasing TiN or Ti(C, N) andthe stable oxide film is likely formed on the surface. Therefore, the Ticontent is restricted to 0.01% or less. More preferably, the content is0.008% or less.

Nb: 0.01% or less

When Nb is exposed to a high temperature equal to or higher than 800° C.such as a solid solution heat treatment temperature, Nb forms δ(Ni₃Nb),which is a stable phase, possibly to cause decrease in strength andlarger variations in strength. Therefore, the Nb content is restrictedto 0.01% or less.

Ta: 0.01% or less

Ta is an element which forms fine compounds with B, C or N to make prioraustenite grain size finer in solid solution treatment after coldmachining, which contributes to reinforcement. Further, Ta is expectedto have an effect of inhibiting skin roughness while decreasingtoughness. Therefore, the Ta content is restricted to 0.01% or less.

W: 0.01% or less

In the same matter as Ta, W forms compounds and causes decrease intoughness. Therefore, the W content is restricted to 0.01% or less.

Next, the hardness defined in the present invention will be described.

Surface hardness of the maraging steel is 800 to 1,050 HV by Vickershardness

In the present invention, the term “surface hardness of the maragingsteel” means the hardness generated by the nitride in the nitridingtreatment near the surface of the maraging steel.

In the maraging steel containing Al in the range of 0.6% to 1.3%according to the present invention, it has been observed that if thesurface hardness of the maraging steel is unnecessarily high after thenitriding treatment, the sensitivity to a surface defect is increased tocause decrease in strength and fatigue strength.

Generally, when the material having Al or Cr added therein as in thepresent invention is subjected to the nitriding, nitrides areprecipitated in the proximity of the surface. Owing to hardeningassociated with the precipitation or volume changes associated with theprecipitation of the nitride, a residual stress is generated to increasethe strength and fatigue strength. In particular, the surface hardnesssignificantly affects the strength and fatigue strength. In a materialhaving no defect, the strength and fatigue strength increase as thehardness increases.

However, industrially produced steels or their products have more orless defects. Therefore, higher hardness conversely causes decrease instrength and fatigue strength. In particular, the surface hardness iscorrelated to the sensitivity to a surface defect having a size ofseveral μm, which are inevitably generated during production. As thesurface hardness increases, the sensitivity to the surface defectincreases. Therefore, the surface hardness of the maraging steelaccording to the present invention is set to 1050 HV or less by Vickershardness. Meanwhile, if the surface hardness is less than 800 HV, it isdifficult to use the maraging steel for a metal endless belt for a CVTor the like, which requires a wear resistance. Therefore, the lowerlimit of the surface hardness is set to 800 HV.

Internal hardness of the maraging steel is 570 HV or less by Vickershardness

In the present invention, the term “internal hardness of the maragingsteel” means the hardness at a position apart from the surface of themaraging steel by such an extent that the position is not subject to theinfluence of the nitriding treatment on the above-described surface.

The internal hardness of the maraging steel is generally regulated to beabout 570 HV in order to obtain sufficient strength and fatiguestrength. However, in the maraging steel with no Ti according to thepresent invention, the internal hardness is obtained as a result of theprecipitation of the intermetallic compounds of Al, such as NiAl orNi₃Al. These intermetallic compounds, such as NiAl, significantlycontribute to increase the strength. However, as the precipitationamount increases, the hardness becomes higher to increase thesensitivity to the internal defect, similar to the above-describedsensitivity to the surface defect. This results in increase in thesensitivity to an internal damage originating from the inclusions, whichoccur in the very high cycle range where the repetition number in thefatigue test is 10⁷ or more. Therefore, the internal hardness of themaraging steel according to the present invention is set to 570 HV orless by Vickers hardness. Meanwhile, the lower limit is not particularlylimited. However, the lower limit is preferably 490 HV since thestrength is decreased if the internal hardness is less than 490 HV. Amore preferable lower limit of the internal hardness is 500 HV.

The effect of each element other than the above-described elements inthe present invention will now be described.

C: 0.008% or less

C forms carbides with Mo to reduce the intermetallic compounds to beprecipitated and decrease the strength. It is therefore required to keepthe C content as low as possible. Further, if C is excessively added,the risk of reducing weldability is high. The weldability is requiredwhen the maraging steel is used for the metal endless belt for a CVT,for example. For this reason, the C content is set to 0.008% or less. Amore preferable upper limit of the C content is 0.006% and a furtherpreferable upper limit is 0.005%.

Si: 0.5% or less

Si is an element which can compensate the decreased amount of thestrength due to reduction of Ti, by making intermetallic compoundsprecipitated in aging treatment finer or by forming intermetalliccompounds with Ni. However, in order to ensure good toughness andductility, it is advantageous to keep the Si content as low as possible.Therefore, in the present invention, the Si content is set to 0.5% orless. A more preferable range for more reliably ensuring sufficienttoughness and ductility is 0.1% or less. More preferably, the range is0.05% or less.

Mn: 0.5% or less

Mn forms intermetallic compounds with Ni in the aging treatment tocontribute to the age hardening. Therefore, Mn is an element which cancompensate the decreased amount of the strength due to reduction of Ti.However, in order to ensure good toughness and ductility, it isadvantageous to keep the Mn content as low as possible. Therefore, inthe present invention, the Mn content is set to 0.5% or less. A morepreferable range for more reliably ensuring sufficient toughness andductility is 0.1% or less. More preferably, the range is 0.05% or less.

Cr: 0.5 to 1.5%

Cr is an element having a high affinity to N in performing the nitridingand reduces the nitriding depth to increase the nitriding hardness andincrease the compressive residual stress on the nitrided surface.Therefore, Cr is added as an essential element. However, if the Crcontent is less than 0.5%, its effect is not significant. Meanwhile,even if Cr is added in an amount of more than 1.5%, its effect is notremarkably enhanced, while the strength after the nitriding treatment isdecreased. Therefore, the Cr content is set to 0.5 to 1.5%. A morepreferable range of the Cr content is 0.8 to 1.2%.

Ni: 16.0 to 22.0%

Ni has an effect of stably forming a low-C martensitic structure whichis a base structure of the maraging steel, and an effect of formingintermetallic compounds with Al to contribute to improve the strength.Therefore, at least 16.0% of Ni is necessary. However, if the Ni contentis more than 22.0%, an austenitic structure is stabilized and themartensitic transformation tends not to occur. Therefore, Ni is set to16.0 to 22.0%. A more preferable range of Ni is more than 18.0% andequal to or less than 21.0%.

Co: 3.0 to 7.0%

Co is an important element for promoting precipitation of the fineintermetallic compounds containing Mo and Al and contributing toenhancement of the aging precipitation, by increasing the solidsolubility of elements forming the aging precipitation such as Mo and Alat the solid solution treatment temperature, while decreasing the solidsolubility of Mo, Al in the aging precipitation temperature range,without significantly affecting the stability of the martensiticstructure in the matrix. Therefore, it is required to add a large amountof Co in terms of the strength and toughness. If Co is less than 3.0%,it is difficult to obtain sufficient strength in the maraging steelcontaining reduced Si, Mn and Ti. Meanwhile, if Co is added in an amountof more than 7.0%, the strength is conversely reduced due to increase insensitivity to the surface defects associated with increase in hardness.Therefore, Co content is set to 3.0 to 7.0%. A more preferable range ofCo is more than 4.0% and equal to or less than 6.0%.

Mo: 3.0 to 7.0%

Mo is an important element which forms fine intermetallic compounds suchas Ni₃Mo and Fe₂Mo in the aging treatment and contributes to enhancementof the precipitation. In addition, Mo is an element effective forincreasing the surface hardness and the compressive residual stress bythe nitriding. If Mo is less than 3.0%, the tensile strength isinsufficient for this purpose. On the other hand, if Mo is more than7.0%, Mo tends to form coarse intermetallic compounds containing Fe andMo as main elements. Therefore, Mo is set to 3.0 to 7.0%. A morepreferable range of Mo is more than 4.0% and equal to or less than 6.0%.

So far, the elements defined as essential elements in the presentinvention have been described. Next, elements which may be optionallyadded will be described.

Mg: 0.0001 to 0.0030%, Ca: 0.0001 to 0.0030%

In the present invention, 0.0001 to 0.0030% of Mg and 0.0001 to 0.0030%of Ca may be contained by mass %.

From the maraging steel according to the present invention, ingots canbe produced by a melting process in a vacuum environment, such as vacuuminduction melting, or vacuum induction melting followed by vacuum arcremelting or electroslag remelting. However, even though such melting inthe vacuum environment is performed, it is technically difficult tocompletely eliminate inclusions.

In the case of the present invention, since Al is added for the purposeof improvement in strength, there are a risk of formation of rough,large and hard Al₂O₃ inclusion having a size of more than 25 μm, forexample, and a risk of clustering of Al₂O₃. The Al₂O₃ inclusion is hardand has a high melting point so that it is hardly transformed evenduring a hot plastic processing, for example. Therefore, the Al₂O₃inclusions can produce a flaw on a roll in cold rolling, for example, togenerate the surface defect in the maraging steel. It is thus preferablethat the Al₂O₃ inclusion is combined with other oxides to form compositeinclusions so as to reduce the hardness or reduce the melting point.Further, at the same time, elements which can prevent clustering arepreferably added in order to prevent inclusion defects.

Si, Mn, Ca and Mg are raised as elements effective for combining theAl₂O₃ inclusions into the composite inclusions. However, in the presentinvention, additive amounts of Si and Mn are restricted because they areelements which cause decrease in toughness and ductility. Therefore,either Ca or Mg, or both thereof other than Si and Mn are preferablyadded to combine the Al₂O₃ inclusions into the composite inclusions.Further, Ca and Mg have an effect of preventing the clustering of theAl₂O₃ inclusions. Therefore, in the present invention, it ispredetermined to contain 0.003% or less of Ca or additionally 0.003% orless of Mg.

Here, in order to reliably obtain the effect of Ca and Mg, it ispreferable that the lower limit of the Ca content is 0.0001% and thelower limit of the Mg content is 0.0001%. Moreover, Mg is expected tohave an effect of making oxide inclusions and nitride inclusions finer.Therefore, Mg is preferably selected when the maraging steel accordingto the present invention is applied to the metal endless belt for a CVT,wherein it is required to prevent fatigue fracture originating from theinclusions, for example. Of course, both Mg and Ca may be jointly added.

B: 0.0001 to 0.0030%

B is an element which contributes to reinforcement by making the prioraustenite crystal grains finer in performing the solid solutiontreatment after the cold processing, and B has also an effect ofinhibiting the surface skin roughness. So, B may be added as required.The B content is set to 0.0030% or less because the toughness isdecreased if the B content is more than 0.01%. More preferably, thecontent is 0.001% or less. A preferable lower limit of the B content is0.0001%, above which it is possible to reliably make the prior austenitecrystal grains finer.

Furthermore, elements other than the above-described elements are Fe andimpurities.

Although it is preferable that the contents of impurities are lower, itis no problem when the contents are within the following range.

-   -   P≦0.05%, S≦0.05%, N≦0.005%, O≦0.003%

The maraging steel according to the present invention containssubstantially no Ti forming the stable oxide film, which may inhibit thenitriding, on the surface. Therefore, a variety of nitriding treatmentsuch as normal gas nitriding, gas soft nitriding, nitrosulphurizing, ionnitriding, salt bath nitriding or the like can be easily performed.

For example, if the maraging steel is machined to have a thickness equalto or less than 0.5 mm and is used for the metal endless belt for a CVT,an absolute value of the compressive residual stress of the nitridedlayer, which tends to decrease in the maraging steel having acomposition without Ti, can be increased by means of Cr or Al having aneffect of increasing the nitriding hardness and the absolute value ofthe compressive residual stress of the nitrided layer.

The maraging steel according to the present invention is expedient tothe metal endless belt for a CVT, because it has a high tensilestrength, a high fatigue strength and excellent fatigue characteristicsowing to the nitriding treatment.

Examples

The present invention will be described in more detail in the followingexamples.

Seven kinds of steel blocks, each having a weight of 10 kg, were createdusing a vacuum induction melting furnace. In order to reliably preventincorporation of Ti, Nb, Ta and W that are elements to be restricted,alloy raw materials having a high purity were employed and added.

The created steel blocks were subjected to homogenizing anneal followedby hot forging. Then, the steel blocks were subjected to hot-rolling andcold-rolling to create the maraging steel having a thickness of about0.2 mm for a metal endless belt for a CVT.

Chemical compositions of the resulting maraging steels and Al/Ti ratiosare shown in Table 1. Every maraging steels were regulated to have0.008% or less of C, in order to prevent decrease in weldability. Itshould be noted that the maraging steels of Nos. 1, 2 and 7 have an Alcontent out of the range in the present invention and, in particular,No. 7 has also an Al/Ti ratio out of the range in the present invention.

Further, as impurities not shown in Table 1, 0.002% or less of P and0.001% of S were present in each of the maraging steels.

TABLE 1 (mass %) Steel No. C Si Mn Cr Ni Mo Co Al Mg Ca B 1 0.004 0.010.01 1.0 18.9 4.9 5.1 1.47 0.0003 — — 2 0.002 0.01 0.01 1.0 19.1 4.9 5.11.35 0.0026 — 0.0006 3 0.004 0.01 0.01 1.0 19.1 4.9 5.1 1.19 0.00200.0003 — 4 0.004 0.01 0.01 1.0 19.1 5.0 5.1 1.01 0.0026 0.0005 — 5 0.0040.01 0.01 1.0 19.1 5.0 5.1 0.81 0.0028 0.0003 — 6 0.003 0.01 0.01 1.019.1 5.0 5.1 0.63 0.0022 — 0.0009 7 0.004 0.01 0.01 1.0 19.1 5.0 5.10.44 0.0011 — 0.0006 Steel No. N O Ti Nb Ta W Al/Ti Balance 1 0.00040.0007 0.002 ≦0.01 ≦0.01 ≦0.01 735 Fe and inevitable impurities 2 0.00040.0006 0.002 ≦0.01 ≦0.01 ≦0.01 675 Same as above 3 0.0005 0.0004 0.002≦0.01 ≦0.01 ≦0.01 595 Same as above 4 0.0005 0.0005 0.002 ≦0.01 ≦0.01≦0.01 505 Same as above 5 0.0005 0.0004 0.002 ≦0.01 ≦0.01 ≦0.01 405 Sameas above 6 0.0005 0.0004 0.002 ≦0.01 ≦0.01 ≦0.01 315 Same as above 70.0008 0.0005 0.002 ≦0.01 ≦0.01 ≦0.01 220 Same as above

The maraging steels shown in Table 1 were subjected to solid solutiontreatment at a temperature of 900° C. in a hydrogen environment and thenaging treatment at a temperature of 480° C., followed by gas nitridingtreatment in a temperature range of 450 to 500° C. as nitridingtreatment. Here, in the gas nitriding treatment, three levels of surfacehardness of the maraging steels were provided to determine the effectsof the Al content and the surface hardness.

From the maraging steels after the nitriding treatment, test pieces forinternal hardness measurement, surface hardness measurement, tensiletest and fatigue strength measurement were sampled and the tests wereperformed. For the internal hardness measurement, the test pieces weresampled from the center in the width direction of the maraging steelsand cut planes of the test pieces were polished. An indentation isapplied on the center in the thickness direction of the test pieces tomeasure Vickers hardness. For the surface hardness measurement, anindentation is applied on the surface of the center in the widthdirection of the maraging steel to measure Vickers hardness.

The results of the internal hardness, the surface hardness, the tensilestrength and the fatigue test after the nitriding treatment are shown inTable 2. Although there are a variety of types of stress loading in thefatigue test, such as rotary bending, tensile and compression, twistingand the like, an evaluation method of applying bending stress issuitable for the maraging steel according to the present inventionbecause it is a strip material. Therefore, in a cyclic bending fatiguetest, repetition numbers until breakage were counted with cyclic bendingstresses applied with an average stress of 679 MPa and a maximum stressof 1300 MPa. The levels of the average stress and the maximum stresswere set so as to make a clear difference between the maraging steels.If the average stress is less than 679 MPa or the maximum stress is lessthan 1300 MPa, it is difficult to break the maraging steels. On theother hand, if the average stress is more than 679 MPa or the maximumstress is more than 1300 MPa, the maraging steels are prematurely brokenso that a fatigue limit, that is an index of fatigue characteristics, isnot clear. For this reason, the stress levels of the repetition bendingtest were set to the average stress of 679 MPa and the maximal stress of1300 MPa.

Further, the cut planes of the maraging steels of No. 1 to No. 6 wereobserved with an electron microscope in 10 fields of view and with amagnitude of 1,000. The Al₂O₃ inclusion was not observed.

TABLE 2 Breakage Al Internal Surface Tensile repitition Steel contenthardness hardness strength number No. (mass %) (HV) (HV) (MPa) (counts)remarks 1 1.47 589 1062 1447 29200 Comparative example 577 931 149475233 Comparative example 569 804 1460 104300 Comparative example 2 1.35581 1100 1657 9933 Comparative example 570 1092 1636 22633 Comparativeexample 573 1038 1747 21700 Comparative example 3 1.19 544 1061 155244933 Comparative example 551 1011 1636 126767 Present invention 541 9411977 914833 Present invention 4 1.01 538 1033 1444 38700 Presentinvention 557 1004 1756 65167 Present invention 523 933 1836 957700Present invention 5 0.81 508 1037 1444 68433 Present invention 504 9921756 316000 Present invention 504 903 1836 1958800 Present invention 60.63 503 998 1439 248433 Present invention 494 986 1606 265400 Presentinvention 491 861 1860 276700 Present invention 7 0.44 472 1030 1512175267 Comparative example 484 998 1757 283900 Comparative example 480836 1812 745633 Comparative example

The results of the internal hardness, the surface hardness, the tensilestrength, the breakage repetition number in the fatigue test of themaraging steels according to the present invention and comparativeexamples show that the tensile strength and the breakage repetitionnumber are changed depending on the surface hardness and the internalhardness in the maraging steels according to the present invention. Inparticular, it is found that the tensile strength and the breakagerepetition number increase as the surface hardness is reduced.

However, in the maraging steel of No. 3 having the Al content close tothe upper limit predetermined in the present invention, an only slightincrease in the surface hardness leads to increase in the sensitivity toa steel surface defect and consequently the breakage repetition numberis as small as 44933.

Further, in the maraging steels of No. 3 to No. 6 according to thepresent invention, the breakage repetition numbers are high. Moreover,the hardness is changed depending on the Al content. In particular, themaraging steels of No. 3 to No. 5 according to the present inventionshow optimal results of the internal hardness of 500 to 570 HV. Themaraging steel of No. 6 according to the present invention has the Alcontent close to the lower limit in the present invention. Thus, itsinternal hardness is relatively low.

Meanwhile, it is found that in the comparative example of No. 1 having ahigh Al content, the tensile strength and the breakage repetition numberare reduced when the internal hardness is as high as 589 HV and surfacehardness is as high as 1062 HV, for example. Moreover, the steel in thecomparative example of No. 7 has a small Al content and therefore theamount of the intermetallic compounds increasing the internal hardnessis small. As a result, the internal hardness is as low as 490 HV or lessin each case, which may represent a problem when this maraging steel isapplied to members that are required to have sufficient hardness, suchas a power transmission metal belt. From the foregoing, it is found thatthe characteristics are significantly changed depending on the Alcontent and there is a suitable range of the Al content.

The comparative example of No. 7 has a low Al/Ti ratio of 150 andformation of AlN was hardly observed when observing the nitrided layerwith a transmission electron microscope. Therefore, the comparativeexample of No. 7 had a low breakage repetition number in the fatiguetest.

Next, in order to further confirm the effect of Al, the tensile test wasperformed after the nitriding treatment of the same batch in nitridingtreatment condition aiming to a surface hardness of 850 HV. The resultsare shown in FIG. 1. FIG. 1 is a view showing the results of the tensiletest of three samples for each maraging steel. Black squares designateaverage values and vertical lines designate variations in measuredvalues.

As can be seen from the maraging steel of No. 1 containing more than1.3% of Al in FIG. 1, variations in tensile strength are salient even ifhardness is regulated. As the Al content is increased, formation of theintermetallic compounds is promoted in the aging treatment to causeincrease in hardness. As a result, the increased Al content is believedto be a cause of variations in tensile strength and fatigue strengthcharacteristics. Further, it is found that variations can also besuppressed with an Al additive amount predetermined in the presentinvention.

Moreover, FIG. 1 shows that if Al is added in an amount of more than1.2%, variations in strength are increased and the tensile strength isdecreased. On the other hand, it is found from the results in Table 2that, when the Al content was in the range of 0.7 to 1.2% and a suitablenitriding condition was achieved (for example, the surface hardness of900 to 950 HV), the breakage repetition number of more than 950,000 wasobtained so that the sensitivity to the steel surface defects wasregulated to be suitable and an excellent fatigue strength was obtained.

In terms of tensile strength, variations in strength and fatiguestrength, the maraging steel having particularly excellentcharacteristics can be obtained by regulating the Al content to be inthe range of 0.7 to 1.2%.

As described above, it is found that high strength, high fatiguestrength characteristics and stable strength characteristics with smallvariations can be obtained by setting suitable levels of internalhardness and surface hardness by the aging treatment and the nitridingtreatment, while optimizing the composition.

INDUSTRIAL APPLICABILITY

The maraging steel according to the present invention is expedient to ametal endless belt for a CVT because it has a high strength and anexcellent fatigue strength.

1. Maraging steel subjected to nitriding treatment, the maraging steelcontaining 0.008% or less of C, 0.5% or less of Si, 0.5% or less of Mn,0.5 to 1.5% of Cr, 16.0 to 22.0% of Ni, 3.0 to 7.0% of Co, and 3.0 to7.0% of Mo by mass % and having a thickness of 0 5 mm or less, whereinthe maraging steel further contains 0.6 to 1.3% of Al by mass %, and thebalance consisting of Fe and impurities, the impurities are restrictedto 0.01% or less of Ti, 0.01% or less of Nb, 0.01% or less of Ta, and0.01% or less of W, a ratio of Al to Ti satisfies Al/Ti≧250, and asurface hardness of the maraging steel is 800 to 1,050 HV and aninternal hardness of the maraging steel is 570 HV or less by Vickershardness.
 2. The maraging steel according to claim 1, wherein the Alcontent is 0.7 to 1.2% by mass %.
 3. The maraging steel according toclaim 1, wherein the maraging steel further contains, in addition tosaid compositions, either 0.0001 to 0.0030% of Mg or 0.0001 to 0.0030%of Ca, or both thereof, by mass %.
 4. The maraging steel according toclaim 1, wherein the maraging steel further contains, in addition tosaid compositions, 0.0001 to 0.0030% of B.